Kaposi's sarcoma-associated herpesvirus vCyclin open reading frame contains an internal ribosome entry site

We have previously examined the transcription and splicing of open reading frames (ORFS) 71 (K13), 72, and 73 of Kaposi's sarcoma-associated herpesvirus (KSHV) in the primary effusion lymphoma cell line BCP-1 (latently infected with KSHV) (45). The three genes encoded by these ORFs (for vFLIP, vCyclin, and latency-associated nuclear antigen [LANA]) are transcribed from a common transcription start site in BCP-1 cells during both latency and the lytic cycles. The resulting transcript is spliced to yield a 5.32-kb message encoding LANA, vCyclin, and vFLIP and a 1.7-kb bicistronic message encoding vCyclin and vFLIP. To investigate whether the vFLIP protein could be expressed from this vCyclin/vFLIP message, we utilized a bicistronic luciferase reporter system. The genes for Renilla and firefly luciferases (which utilize different substrates) were cloned in tandem downstream from a T7 RNA polymerase promoter. Fragments of DNA immediately upstream from the initiating codon of vFLIP were cloned between the two luciferase genes. The relative expression of the two luciferases, one directed by the putative internal ribosome entry site (IRES) sequences and the other by cap-dependent ribosome scanning, was used to compare the activities of the different DNA fragments. A minimum fragment of 233 bp within the coding region of vCyclin was found to direct efficient expression of the downstream cistron (firefly luciferase). The activity of this IRES was orientation dependent and unaffected by methods used to inhibit cap-dependent translation. This is the first demonstration of an IRES element encoded by a DNA virus and may represent a novel mechanism through which KSHV controls protein expression.